In an example of electrophotography, an electrostatic latent-image is formed on a photoconductive photoreceptor by charging the photoreceptor and then exposing it to light. The resulting electrostatic latent-image is developed with a developer so that a toner-image can be formed. Then, the resulting toner-image is transferred to an image-transfer material and the transferred image is then so fixed as to form a copied-image.
In such an image-forming process as in the above-described electrophotography, there raises such a problem as will be mentioned below.
(1) Environmental dependency of image-transfer characteristics PA1 In an image-transfer step of an image-forming process, the image-transfer characteristics of the image-transfer process tend to depend greatly on the environment. Particularly when an image is formed under high-temperature and high-humidity conditions, an image-transfer ratio to an image-transfer material is relatively lower than in the case that an image is formed under the conditions of an ordinary temperature and an ordinary humidity, and an image-transfer unevenness may sometimes produced on a resulting copied-image, so that any satisfactory image-transfer characteristics cannot be enjoyed. PA1 Because of the above-mentioned reasons, the charged amount of toner, that greatly influences image-transfer characteristics, cannot become satisfactory and an image-transfer material absorbs moisture and the surface of the image-transfer material is dew-condensed, so that an electric resistance on the surface of the image-transfer material is extremely lowered thereby and, especially, under the conditions of a high-temperature and a high-humidity. PA1 (2) Environmental dependency of image-density
A copied-image density tends to depend greatly on the environment and, particularly, a humidity environment (or an moisture in the air) greatly influences image-densities.
For example, when a copied-image of a half-toned gray-chart is formed under the conditions of a high-temperature and a high-humidity, the image-density of a copied-image to be obtained is made higher than the original density, so that any fidelity image-density cannot be reproduced, because the charged amount thereof is lowered under the conditions of a high-temperature and a high-humidity, so that an over-development cannot be avoided.
On the other hand, from the viewpoints of the improvement of toner chargeability and the environmental safety of charging amount, the techniques for containing fluororesin particles in toner -Refer to Japanese Patent Open to Public Inspection (hereinafter referred to JP OPI Publication) Nos. 53-130036/1978, 62-75539/1987, 62-75553/1987 and 3-170946/1990.
According to these techniques, the environmental safety of charging amount can be attempted by making use of hydrophobic fluororesin particles and thereby the above-mentioned problems (1) and (2) can be solved to some extent.
However, the fluororesin particles applicable to the techniques described in the above-given patent specifications are comprised of a (co-)polymer having a non-cross-linked structure and containing fluorine atom at the principal chain thereof. The fluororesin particles comprising such a (co-)polymer as mentioned above are so soft that they are liable to be deformed. Therefore, when making use of a toner containing the fluororesin particles such as those mentioned above and images are formed repeatedly through a copying machine loaded with a toner recycle system or the like, the fluororesin particles are made adhered to and fused into a developing sleeve and/or a carrier so as to stain them. Resultingly, a white streak is produced on a copied image to be formed and the copying machine is stained inside by flying the toner therein.
In the techniques described in the above-given patent specifications, as mentioned above, the durability of the particles are shortened as a developer, so that a series of multicopied images cannot stably be formed.
When making use of a two-component type developer comprising toner and carrier, the above-mentioned problem (2) cannot satisfactorily be solved even when making use of the techniques described in the above-given patent specifications. To be more concrete, even if a hydrophobic resin is introduced only into toner, the environmental safety of an image-density cannot satisfactorily be achieved, because a charged amount of toner contained in a two-component type developer is a physical amount effective in an interaction between toner and carrier.